SQL查询：每组单行

Question

我正在尝试为每个组获得一个结果，但它返回的不止一个。在这里我的发言

SELECT in_id, in_name, ig_id, ig_name, di_name, iv_id, iv_reading, iv_date, iv_time
FROM `instruments`
JOIN instrument_group ON ig_id = in_igid
LEFT JOIN instrument_reading ON iv_inid = in_id AND date(iv_date) = date(20130301)
RIGHT JOIN dam_info ON di_id = ig_diid
WHERE ig_id = in_igid

这里是输出

如上所示，in_id 6有两个结果，我只想要一个表instrument_reading的最新结果。

更新：表架构

非常感谢您的帮助。

Answer 1

SELECT in_id, in_name, ig_id, ig_name, di_name, iv_id, iv_reading, iv_date, iv_time
FROM `instruments`
JOIN instrument_group ON ig_id = in_igid
LEFT JOIN (SELECT iv_id, iv_inid, iv_reading,iv_date, iv_time FROM instrument_reading     WHERE date(iv_date) = date(20130301) ORDER BY iv_time DESC LIMIT 1) as latestDt ON latestDt.iv_inid = in_id
LEFT JOIN dam_info ON di_id = ig_diid
WHERE ig_id = in_igid

我用它作为临时解决方案。如果你有更好的方法，我会打开你的